1. Field of the Invention
This invention relates to pallets and especially relates to warehousing pallets used for supporting loads to be moved by lift trucks. It particularly relates to devices that prevent sidewise movement of very heavy loads on flexible pallets, such as plastic pallets.
2. Review of the Prior Art
Pallets serve as a tray-like support for the stacking and transport of articles and generally comprise lateral openings which can be engaged by the arms of fork-lift devices. Four-way pallets have these lateral openings on all four sides, and double-deck pallets have these openings between an upper deck and a lower deck. More simple constructional forms have only a single upper deck with base portions which are arranged therebeneath and between which the arms of a fork-lift truck are inserted. Box pallets essentially comprise a box-like side wall arranged around the load-supporting surface of a flat pallet.
Furthermore, it is desirable for the lateral openings of a pallet to have sufficient width and to be open towards the ground beneath the load-carrying deck of the pallet so that the arms of a fork-lift device can fit thereinto and a fork-lift truck can drive into the lateral openings and can then engage its arms underneath the pallet while having its wheels on the ground.
A feature common to all pallets is that they are intended for accommodating loads and consequently have to be constructed in a very robust and load-supporting manner, depending on the nature of the loads. This robustness can only be achieved with difficulty by means of metal pallets and, to a certain degree, also by means of wooden pallets; however, it is desirable to utilize substitute materials for reasons of economy, but problems arise in attempting to meet standards as to stability and permanent strength. With pallets made of plates, flexing becomes an acute problem when supporting very heavy loads.
These problems become particularly clear when considering standardized flat pallets for bottle crates, which have a standard size of 1000 x 1200mm, as an example of a very heavy load. Such bottle crates may be placed in one layer, in a 3 .times. 3 pattern, on the corresponding load-bearing surface of such a pallet. Furthermore, it is usual for five layers of bottle crates to be stacked one above the other on the pallet.
A synthetic plastic bottle crate when empty weighs about 2 kg, and in each bottle crate there are arranged, for example, 20, 0.5-liter bottles or 24, 0.33-liter bottles. A single bottle crate with full bottles then weighs between about 18 and 22 kg, or roughly calculated 20 kg. Consequently, one layer of bottle crates weighs 180 kg and five layers weigh 900 kg, that is to say, almost 1 ton. It is consequently understandable, in view of the entirely different properties of metals and plastics as regards modulus of elasticity, tendency to non-elastic deformation, and the like, that flat pallets made of synthetic plastic materials have not yet been usable in practice for supporting bottle crates and similar heavy loads.
Suitable synthetic plastics materials for the construction of pallets are, for example, polyolefins, such as polyethylene and polypropylene, ABS polymers, and polyurethanes. However, if very heavy loads, such as coils of steel, counter rolls of paper, and bottle crates, are loaded on a flexible pallet made of such plastics, the pallet may flex or bend sufficiently to enable the load to move or slip sidewise, thereby causing all or part of the load to fall off the pallet or causing the lift truck to capsize. Obviously such slippage is potentially very dangerous and expensive.
In the prior art, a raised rim around the load-bearing surface of the pallet has hitherto been provided for stopping any lateral slipping off of the load from the pallet. However, a load may be more or less centered upon the pallet and not be close to the rim. Accordingly, a laterally moving pallet may gather considerable momentum before contacting the rim or may slide laterally sufficiently to cause severe unbalancing of the entire pallet. This effect can be particularly serious if the pallet is thereupon stressed beyond its elastic limit so that bending strain is thereafter not in proportion to stress.
This effect may also not be perceivable under static or carefully supervised operating conditions, but when fork-lift trucks are normally rushing down warehouse aisles, around corners at high speeds, and suddenly stopping at intersections, within a boxcar, etc., the momentum of the load can generate severe laterally directed forces under such accelerative and/or decelerative conditions. Consequently, a severe downward force can be created at the outward edge of the pallet (relative to the direction of the accelerative or decelerative movement) as the resultant of a couple developed from lateral momentum at the center of gravity of the load and frictional resistance at the bottom thereof. This downward force can momentarily cause extraordinarily great flexing of the plastic pallet, particularly if the elastic limit of the plastic material is exceeded; this flexing necessarily acts in combination with the momentum-derived lateral force acting on the pallet, whereby ordinarily unlikely sidewise slippage can occur.
A device is accordingly needed for improving flexible pallets so that any lateral movement of a very heavy load upon the downwardly flexed load-bearing surface thereof is prevented or is substantially inhibited before the encircling rim is contacted by the load.